1. Field of the Invention
The invention is directed to a polymerization process and catalyst system for the production of polyphenylene ethers of controlled molecular weight. Polyphenylene ethers made by this process have improved flow and other advantageous properties, and are useful as components of thermoplastic molding blends.
2. Brief Description of the Prior Art
The polyphenylene ethers and processes for their preparation are known in the art and described in numerous publications, including Hay, U.S. Pat. Nos. 3,306,874 and 3,306,875. Other procedures are described by Van Dort, U.S. Pat. No. 3,365,422, Bennett and Cooper, U.S. Pat. Nos. 3,639,656, 3,642,699, 3,733,299, 3,838,102, 3,661,848 and 4,092,294, and Olander, U.S. Pat. No. 4,083,828. All of these patents are incorporated herein by reference.
The processes most generally used to produce the polyphenylene ethers involve the self-condensation of a monohydroxy phenol in the presence of an oxygen-containing gas and a catalyst, generally a metal salt or complex.
Improved catalyst systems have been developed for the preparation of polyphenylene ethers by the oxidative coupling of 2,6-di-substituted phenolic compounds. Such systems are disclosed in the above-noted U.S. Pat. No. 4,092,294. This patent discloses a catalyst which comprises a copper compound; a diamine wherein the two amino nitrogens are separated by at least two and no more than three carbon atoms, and the carbon atom to which the amino nitrogen is attached is aliphatic; a tertiary amine and a bromine-containing compound selected from the group consisting of hydrogen bromide, alkali metal bromides, alkaline earth metal bromides, 4-bromophenols and mixtures thereof plus an amount of a primary or secondary monoamine, most frequently a secondary monoamine of the formula RNHR' wherein R and R' are lower alkyl.
The use of some of the known catalyst systems for preparing polyphenylene ethers results in the incorporation of the primary or secondary monoamine into aminomethyl groups located not only at the terminal phenol of the polymer chain but also at various non-terminal phenylene ether units along the chain. Thus no simple stoichiometric ratio exists between the number of molecules of the amine used and the number of polymer chains formed. Moreover, these multiple aminomethyl groups may serve as a source of odoriferous breakdown products during thermal processing of the polyphenylene ether such as in preparation of moldable blends.
An alternative process for polyphenylene ethers has been described by Percec and Shaffer, J. Polym. Sci.: Part C: Polymer Letters, vol. 24, 439-446 (1986). In this process, 4-bromo-2,6-dimethylphenol is polymerized in the presence of toluene, aqueous sodium hydroxide, a phase-transfer catalyst (tetrabutylammonium hydrogen sulfate) and air. Although this process avoids the use of metals, it still does not afford effective means for molecular weight control.
The processes described above all have the disadvantage of not affording a convenient means for reliably attaining any predetermined molecular weight. It is desired for example to be able to limit the molecular weight in a controlled manner so as to obtain polyphenylene ethers which have improved flow properties, and which are easier to blend with other polymers. Typical attempts to control the molecular weight of polyphenylene ethers have involved changing the catalyst-to-monomer (phenol) ratio, the monomer feed rates, and the rate of oxygen uptake. Other attempts have focused on adding a nonsolvent to the reaction, thereby precipitating the polymer at a certain molecular weight. These methods are tedious, give inadequate control, and often lead to polymer recovery problems.
The products made according to the above-cited known copper and manganese-catalyzed processes, moreover, tend to be contaminated by side products of the reactions, namely 3,3',5,5'-tetraalkyl-4,4'-diphenoquinone, a colored compound. This side product usually gets incorporated to some degree into the polymer chain, thus leading to polymer chains with two phenolic ends, further complicating the product composition, causing broadened and less reproducible molecular weight distributions. Moreover, the products made by the above-described process often have traces of heavy metal compounds (catalyst residues) as impurities, further impairing color, stability and electrical properties of blends made using these products.
The process of my invention makes available control over molecular weight, and makes available new polyphenylene ethers of controlled molecular weight, controlled nitrogen content and essential freedom from diphenoquinone, from incorporated diphenoquinone structures, and from heavy metal compounds. The process makes available new and improved polyphenylene ethers which have improved and more reproducible flow properties, and which are readily blended with other compatible thermoplastics to afford blends of reduced odor, color, improved stability and improved electrical properties.